Vehicles have been equipped with spike tires or with tires with chains for driving snow- and ice-covered roads. This, however, causes environmental problems such as powder dust pollution, and therefore studless tires have been developed as the replacement for the spike tires and the tires with chains for driving snow- and ice-covered roads. The studless tires have been improved in their materials and designs for driving the snow-covered roads having rougher surfaces compared to the surfaces of usual roads. For example, studies have been made to develop a rubber composition that contains diene rubber having excellent low-temperature properties.
Along with the recent global warming, vehicles with studless tires are driven increasingly on paved roads. For driving on various road surfaces including the paved road surfaces, improvement of both dry performance and wet performance of a tire is demanded. In particular, improvement of the wet performance of a tire is strongly demanded as it affects the safety of driving. On the other hand, since further improvement of the performance on snow and ice of a tire is demanded, the rubber hardness of a cap tread tends to be set so that Tg (glass transition temperature) is low in order to achieve better low-temperature properties. However, it is difficult for the conventional art to provide a rubber composition with both snow performance and wet performance.
In general, in order to improve the wet performance of a rubber composition, for example, the blending amount of fillers such as carbon black and silica is increased, or alternatively, the hysteresis loss of the rubber composition is increased by using a rubber component having a high Tg. However, the above methods decrease the flexibility of the rubber composition at low temperatures (i.e. the hardness of the rubber composition is increased), resulting in deteriorated performance on snow and ice.
Patent Document 1 discloses a rubber composition containing epoxidized polyisoprene, which can enhance abrasion resistance, etc. Patent Document 2 discloses a combination use of a rubber component having a high affinity for white fillers and a rubber component having a low affinity for white fillers in order to unevenly distribute the white fillers in the rubber composition. Here, epoxidized natural rubber is used as one constituent of the rubber component. Patent Document 3 discloses a rubber composition containing epoxidized natural rubber and a fatty acid metal salt, which exerts good heat aging resistance.
However, in Patent Documents 1 to 3, the blending amount of epoxidized natural rubber and its epoxidation degree are not specifically studied. Further, those methods do not sufficiently improve the performance on snow and ice and the wet performance in good balance.
Patent Document 1: JP 2006-188571 A
Patent Document 2: JP 2006-348222 A
Patent Document 3: JP 2008-303332 A